In the past, many machines such as power operated rotary blade lawnmowers, edgers, saws and the like have been provided in which the rotating element or blade has been directly connected to the output shaft of the power plant so that the blade has been driven as long as the power plant was operated. This has presented a constant danger to the operator who sometimes has inadvertently inserted his hand or foot into or under the blade housing where it was struck by the blade and resulted in severe bodily damage to the operator.
Some efforts have been made to provide a centrifugal clutch arrangement intermediate the power plant and the driven element or blade so that the blade is driven only when the power plant output shaft exceeds a predetermined number of revolutions per minute and the blade is freely rotatable relative to the output shaft when the number of revolutions falls below the predetermined number. However, when the power plant has been driven at operating speed and then is reduced to idling speed, the rotating blade continues to free wheel as it slows until it is stopped by friction or the blade strikes an object. An example of this type of arrangement is shown in U.S. Pat. No. 3,058,561 to Spalding.
Some additional efforts have been made to provide a friction clutch which is applied by centrifugal force and in which the rotating element or blade of the lawnmower has been provided with a brake which stops the rotation of the blade either manually or automatically. In such cases, as in U.S. Pat. Nos. 3,026,665 and 4,035,994 to Hoff, and U.S. Pat. No. 3,253,391 to Meldahl, for example, the brake mechanism is attached to an engine governor and such governor is controlled by a manually operated control rod so that the operator can control the speed of the power plant by operating the control rod. Since the brake is attached to the governor when the control rod is operated to reduce the number of revolutions of the output shaft of the power plant, the brake also is controlled by the control rod.
In other structures, such as U.S. Pat. No. 3,871,159 to Shriver; U.S. Pat. No. 4,058,957 to Roseberry; U.S. Pat. No. 4,117,651 to Martin; U.S. Pat. No. 4,122,652 to Holtermann; and U.S. Pat. No. 4,205,737 to Harkness et al, the rotating element is connected to the power plant by a centrifugal clutch and a brake band is connected to a deadman control lever so that the brake is applied as soon as the deadman lever is released.
In still other prior art structures, such as U.S. Pat. No. 3,731,472 to Kamlukin and U.S. Pat. No. 3,994,376 to Fulghum, the rotating element or blade is mounted on a threaded portion of a drive shaft so that when the power plant is operated at a predetermined number of revolutions per minute, the threaded connection causes the blade to be lowered to an operating position and when the driving torque which is applied to the shaft is interrupted, the rotating blade provides a torque which moves the blade upwardly about the threads so that the blade housing engages a braking surface.